Difference between function with returned type pointer and function pointer

Question

I have a code with struct defination with member function pointers like this

    struct file_system_type {
        
        struct dentry *(*mount) (struct file_system_type *, int,
                   const char *, void *);
        void (*kill_sb) (struct super_block *);
        
    };

and object of file_system_type like this

    static struct file_system_type minix_fs_type = {
        .mount      = minix_mount,
        .kill_sb    = kill_block_super,
        
    };

and .mount like this

    static struct dentry *minix_mount(struct file_system_type *fs_type,
     int flags, const char *dev_name, void *data)

I like to know what is the difference of above from function with return type some pointer like if I had something like this

    static struct dentry* minix_mount(...)

Answer 1

struct dentry *(*mount) (struct file_system_type *, int,
               const char *, void *);
void (*kill_sb) (struct super_block *);

are pointers a function that have return type struct dentry * resp. void . First you have to assign an actual function to these pointers to call these functions through these pointer. The pointers in your code are assigned with

    .mount      = minix_mount,
    .kill_sb    = kill_block_super,

static struct dentry *minix_mount(struct file_system_type *fs_type,
 int flags, const char *dev_name, void *data)

is a function that returns a pointer. It already has a static function body and can instantly be called.

Both calls return a value with the same type struct dentry * .

A big advantage of function pointers is that you can write generic code and assign a different functions to this pointer at run time. Common use-cases are algorithms like sort or find algorithms where you can pass a predicate or compare function to a function pointer.

Another advantage is that structs in C can contain function pointers but can't contain functions. That's a way to simulate OOP in C.

Here is an example of a function pointer to a function returning a pointer:

#include <stdio.h>

struct S {
    int x;
    int y;
    // pointers to functions returning a pointer
    int *(*compare1)(int *, int *);
    int *(*compare2)(int *, int *);
};

// functions returning a pointer
int *min(int *a, int *b) {
    return *a < *b ? a : b;
}

int *max(int *a, int *b) {
    return *a > *b ? a : b;
}

int main() {
    struct S s = {
        .x = 5,
        .y = 7,
        .compare1 = &max,
        // .compare2 = &min;
        // name of a function can be used as function pointer
        .compare2 = min
    };
    int *result1 = s.compare1(&s.x, &s.y);
    int *result2 = s.compare2(&s.x, &s.y);
    ++*result1;
    --*result2;
    printf("%d %d", s.x, s.y);
}

Output:

4 8